Optical arrangement for obtaining a measurement signal for power measurement in lasers

ABSTRACT

In an optical arrangement for obtaining a measurement signal for power measurement in lasers, the object is to reliably prevent laser radiation-induced contamination of the optically active surfaces with comparatively simple means, so that the optical properties of the surfaces, in particular the reflection coefficient, remain unchanged for a longer period of time than in the past. A beam splitter that is arranged on the laser beam axis and that directs a fraction of the laser beam onto a sensor by reflection, has optically active surfaces for the reflection that limit an opposing contamination-free, hermetically sealed inner space that is highly transmissive for the wavelength of the laser radiation.

The invention relates to an optical arrangement for obtaining ameasurement signal for power measurement in lasers with a beam splitterthat is arranged on the laser beam axis and that directs a fraction ofthe laser beam onto a sensor by reflex formation on optically activesurfaces.

Important characteristic values in continuously emitting lasers andhighly repetitive pulsed lasers with pulse repetition rates in the kHzrange and higher (quasi-continuous or q-cw) are the power and the meanpower of the laser beam and, generally, pulse energy in pulsed laserswith lower repetition rates.

The goal in laser design is frequently oriented toward measuring thesecharacteristic values in the laser itself, especially the measurementvalues for regulating and stabilizing the laser power or laser pulseenergy.

A power monitor integrated in the laser should use only a small portionof the laser power generated in order not to limit unnecessarily thepower of the usable radiation leaving the laser and should measureprecisely over long periods of time.

For satisfying the first requirement, generally optical beam splittersare placed in the beam path; they split off a fraction (typically a fewpercent or less) of the generated laser beam by transmission orreflection as measurement signal.

If the measurement signal is to be generated by transmission, mirrorsmade of dielectric layer systems are available. While the usefulradiation of high power is reflected, the lower power radiation fractionto be measured penetrates the multilayer system.

Power measurement using this functional principle is disadvantageous interms of long-term stability. Although the optical properties of thelayer system have only a minor dependence on exterior factors such ashumidity, temperature, and power of the striking radiation, which do nothave a significant effect in the highly reflecting radiation portion,these minor changes do lead to substantial fluctuations in the radiationfraction to be measured. This has particularly negative effects when themeasurement signal is used to stabilize the usable laser power.

A measuring device known from DE 43 36 589 C1 with a sensor and anelectronic evaluation and display devices provides for laser powermeasurement a beam splitter that is shielded by screens with a beamentry opening and an exit opening and that is highly transmitting forthe laser wavelength; two beams as fractions of the laser radiation arereflected to the sensor from its beam splitter surfaces.

Even with this technical solution, it is not always possible to satisfythe second requirement, particularly when laser-induced changes in thereflection factor occur at the beam splitter as a main problem for apower measurement that is stable over the long term.

The disadvantage connected with this results from the requirement thatthe measurement signal S received by the sensor should be proportionalto the useful power P emitted by the laser, where S=k×P. Normally thelaser manufacturer calibrates the proportionality factor k by measuringthe external power P with a calibrated power meter. Each additionalcalibration or check of calibration can be associated with higher costsbecause the laser is frequently used in applications in which access foran external power measurement is rendered more difficult because of beamguide optics or other components (e.g., encapsulation of the beam) or isundesirable due to production processes.

Therefore, the proportionality factor k, once calculated, should remainunchanged for the longest periods possible over the service life of thelaser; that is recalibration intervals should be as long as possible.

Particularly in the ultraviolet (UV) spectrum, changes that occur in thetransmission and reflection coefficient due to the formation anddepositing of micro-particles on the optical surfaces, which formationand depositing are induced by the laser itself, are a major problem interms of power measurement that is stable over a long term.

U.S. 2003/0007537 cites as a reason impurities that are in theenvironment of the optical components, that are generally present in thegaseous phase, and that are frequently organic. They can have manydifferent origins, such as e.g. out-gases from materials such as O-ringseals, adhesives, cable insulators, or other sources. The impuritieseven occur when the laser interior is hermetically sealed against theexternal environment.

The suggestion, for reducing impurities of optical components that arehoused in a closed housing in a gas atmosphere, to draw off gas from theatmosphere and send it in multiple successive steps through suitableparticle and active carbon filters, is not satisfactory due to its greatcomplexity, especially since the method requires that the saturation ofthe filters used be monitored continuously.

Starting at this point, it is the object of the invention to improve thearrangement identified in the foregoing such that the laserradiation-induced contamination of the optically active surfaces isreliably prevented using relatively simple means so that the opticalproperties of the surfaces, in particular the reflection coefficient,remain unchanged over a longer period of time than in the past.

In accordance with the invention, the object is achieved in anarrangement of the type identified in the foregoing in that the surfacesthat are optically active for the reflection limit an opposingcontamination-free, hermetically sealed inner space that is highlytransmissive for the wavelength of the laser radiation.

In one preferred embodiment of the invention, it is provided that thecontamination-free, hermetically sealed inner space has opposing beamentry and beam exit windows as space limits, and that their windowsurfaces that face one another form the optically active surfaces.

By constructing a closed contamination-free microvolume that protectsthe optically active surfaces from particle deposits in a uniform andhighly effective manner, a power monitor can be integrated in the laserthat power monitor not only reduces to a small degree the laser powerprovided for use, but that also has a long service life, since thecauses of fluctuations in the reflection coefficient, which particularlyimpact the weak measurement signal, are prevented at their source.

In contrast to housing all of the optical components of a laser in acommon housing, when constructing the microvolume, optics assemblyjoining techniques can be applied that avoid the sources ofcontamination.

Additional advantageous embodiments are explained in greater detailusing the schematic drawings.

FIG. 1 illustrates an optical arrangement for forming a measurementsignal for power measurement in a laser beam.

FIG. 2 illustrates an optical component acting as beam splitter for theoptical arrangement in accordance with FIG. 1 that has an enclosedcontamination-free micro-volume for forming a measurement signal.

The optical arrangement illustrated in FIG. 1 contains, arranged on thelaser beam axis X-X, a beam splitter 1, which directs a fraction of thelaser beam power as a measurement signal to e.g. a sensor 2 that isembodied as a photodiode and that includes evaluation electronics (notshown), while the rest of the laser beam power remains nearly completelyavailable as useful power P. For illustration purposes, only the reflexP′ formed by the front surface of the beam splitter 1 that forms themeasurement signal is illustrated. Beam mixing and/or reducing opticalcomponents such as e.g. diffusers or filters can be arranged between thebeam splitter 1 and the sensor 2, which is symbolized by an illustratedcomponent 3.

In this arrangement, P′=r×P, where r<<1 for the reflection coefficient,so that P′<<P.

For obtaining a measurement signal in the form of a reflex, inaccordance with FIG. 2 an optical component provided as a beam splitter1 for the inventive arrangement has opposing optically active surfaces 4and 5 that limit a contamination-free, hermetically sealed inner space Ithat is highly transmissive for the laser wavelength.

By design, the optically active surfaces 4 and 5 form window surfaces,that face one another, of a beam entry window and a beam exit window 6and 7. The two windows 6 and 7, together with a pair of opposing walls 8and 9, enclose the inner space I, whereby the sealing means used do notform sources of out-gases. Known joining techniques from optics assemblytechnology, such as e.g. friction or diffusion welding can be consideredfor this. For ensuring a contamination-free atmosphere, the inner spaceI is preferably filled with an ultrapure inert gas such as e.g. a noblegas or it is evacuated.

As a rule, laser radiation is linearly polarized so that the opticallyactive surfaces 4 and 5 do not have to be provided with a specialdielectric coating for generating the reflex for the measurement signal.Although changes in the reflection coefficient r that result from adegradation of the coating can be avoided by this, in practice it hasbeen demonstrated that even precluding changes in dielectric coatingsystems and constancy in the refraction coefficient and in the angle ofincidence cannot preclude changes to the reflection coefficient r.Therefore, in accordance with the invention it is suggested that aclosed contamination-free microvolume be constructed that protects theoptically active surfaces 4 and 5 from contamination.

The laser radiation L enters through the beam entry window 6 and leavesthe optical component through the beam exit window 7. If the wavelengthof the laser radiation is in the UV range, both windows 6 and 7 shouldpreferably comprise synthetic quartz glass or CaF2.

What is crucial for the function of the optical component is theoccurrence of a reflex P1′ on the optically active surface 4 during thetransition of the beam entry window 6 to the inner space I and theoccurrence of another reflex P2′ on the optically active surface 5during the transition from the inner space I to the beam exit window 7.

Using a screen, one of the two reflexes P1′ or P2′ can be obstructedwhen it strikes the sensor 2. This measure is illustrated by a screenlabeled 10 between the reflex P1′ and the sensor 2. Another suitablemeasure is applying an anti-reflection coating to one of the opticallyactive surfaces 4 or 5, which suppresses the formation of one of the tworeflexes P1′ or P2′.

Furthermore, the window surfaces 11 and 12, which face away from oneanother, of the beam entry window 6 and beam exit window 7 can beprovided with an anti-reflection coating effective for the laserwavelength, which elevates the overall transmission of the opticalcomponent and prevents the formation of interfering reflexes.

1. An optical arrangement for measuring a laser signal with a sensor,the laser providing an axial beam, the arrangement comprising: a beamsplitter, said beam splitter having first and second optically activesurfaces, said beam splitter being arranged on the laser beam axis, saidbeam splitter partially reflecting the laser beam towards the sensor;and said beam splitter having a container between said optically activesurfaces, said container being contamination-free and hermeticallysealed, said container being capable of transmitting a preselectedwavelength, said wavelength corresponding to a wavelength of the laserbeam.
 2. The arrangement of claim 1, further comprising: beam entry andbeam exit windows, said entry and exit windows being mutually opposingand having opposing internal surfaces, said internal surfaces facing oneanother and defining said optically active surfaces.
 3. The arrangementof claim 2, wherein said beam entry and exit windows have respectiveopposing exterior surfaces, said exterior surfaces facing away from oneanother, said exterior surfaces comprising an anti-reflection coating,said coating blocking said preselected wavelength.
 4. The arrangement ofclaim 1, wherein said beam splitter splits said beam into a plurality ofcomponents and reflects said components towards the sensor, saidarrangement further comprising a screen, said screen preventing at leastone of said plurality of beam components from striking the sensor. 5.The arrangement of claim 1, wherein said beam splitter splits said beaminto a plurality of components and reflects said components towards thesensor, said arrangement further comprising an anti-reflection coating,said coating being located against one of said optically activesurfaces, said coating preventing at least one of said plurality of beamcomponents from striking the sensor.
 6. The arrangement of claim 4,wherein said screen is separate from said beam splitter and the sensor,said screen being arranged between said beam splitter and the sensor. 7.The arrangement of claim 2, wherein said beam entry and exit windowscomprise a UV-permeable material.
 8. The arrangement of claim 1, whereinsaid container comprises an ultra-pure inert gas.
 9. The arrangement ofclaim 1, wherein said container comprises a vacuum chamber.